The Royal Society report ‘Geoengineering the Climate’ identiﬁed solar radiation
management using albedo-enhancing aerosols injected into the stratosphere as the most
affordable and effective option for geoengineering, but did not consider in any detail the
options for delivery. This paper provides outline engineering analyses of the options,
both for batch-delivery processes, following up on previous work for artillery shells,
missiles, aircraft and free-ﬂying balloons, as well as a more lengthy analysis of continuousdelivery systems that require a pipe connected to the ground and supported at a
height of 20 km, either by a tower or by a tethered balloon. Towers are shown not
to be practical, but a tethered balloon delivery system, with high-pressure pumping,
appears to have much lower operating and capital costs than all other delivery options.
Instead of transporting sulphuric acid mist precursors, such a system could also be
used to transport slurries of high refractive index particles such as coated titanium
dioxide. The use of such particles would allow useful experiments on opacity, coagulation
and atmospheric chemistry at modest rates so as not to perturb regional or global
climatic conditions, thus reducing scale-up risks. Criteria for particle choice are discussed,
including the need to minimize or prevent ozone destruction. The paper estimates the
time scales and relatively modest costs required if a tethered balloon system were to
be introduced in a measured way with testing and development work proceeding over
three decades, rather than in an emergency. The manufacture of a tether capable of
sustaining the high tensions and internal pressures needed, as well as strong winds, is
a signiﬁcant challenge, as is the development of the necessary pumping and dispersion
technologies. The greatest challenge may be the manufacture and launch of very large
balloons, but means have been identiﬁed to signiﬁcantly reduce the size of such balloons
or aerostats.
Keywords: geoengineering; climate change; tethered balloons; stratospheric particle injection

*Author for correspondence (cjb@eng.cam.ac.uk).
One contribution of 12 to a Discussion Meeting Issue ‘Geoengineering: taking control of our planet’s
climate?’.

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1. Introduction
Lord Rees commented in the foreword to the Royal Society report on
geoengineering, ‘many proposals for geoengineering have already been made—
but the subject is bedevilled by much doubt and confusion. Some schemes are
manifestly far-fetched; others are more credible, and are being investigated by
reputable scientists; some are being promoted over-optimistically’ [1].
At an Engineering and Physical Sciences Research Council/Natural
Environment Research Council workshop in March 2010, proposals were invited
for preliminary research into geoengineering by various methods including
solar radiation management (SRM) by particle injection into the stratosphere.
At the workshop, the idea of a high-altitude tethered balloon delivery system,
with ultra-high pressure pumping to elevate ﬂuids or particle slurries, appeared to
offer signiﬁcant advantages over other delivery options. A proposal to investigate
the desired particle properties, the method of their delivery and modelling
their impact on the climate was funded. The dispersion at altitude of particles
manufactured at ground level, with tailored size distributions and coatings may
provide beneﬁts, such as reduced or negligible ozone impact, not readily available
to other delivery options. This paper reviews the merits of this idea alongside
those of other delivery options.
Previously, Blackstock et al. [2] considered the scientiﬁc and engineering
requirements of various technologies but did not consider costs. Others have
provided cost estimates for certain technologies such as aircraft and naval
artillery but did not consider as many delivery options [3–5]. Consideration
has also been given by some of the authors to the use of tethered
aerostats, manufactured particles, drag reduction strategies and dispersion
technologies [6].
The lead time required for implementation of any injection system is a
particularly important criterion if the real value of SRM options is to provide an
insurance policy against global warming and its effects: rising greenhouse gas
concentrations may trigger signiﬁcant transients such as runaway methane
emission from melting arctic permafrost, major acceleration of ice sheet melting,
or methane clathrate release from the ocean ﬂoor. Should any of these ‘tipping
points’ be encountered then immediate measures will be required to reduce global
temperature quickly in order to avoid unprecedented social, environmental and
economic costs. For this reason, it is important to assess the time needed for
a candidate geoengineering strategy to have any signiﬁcant impact on global
temperature. Modifying greenhouse gas concentrations is likely to take far too
long: time constants for natural processes to reduce greenhouse gases in the
atmosphere are hundreds of years, and the time constant for man-made emissions
to fall to insigniﬁcant levels is likely to be similar. Carbon dioxide removal
techniques might possibly have an impact in 50 years [7], but SRM technologies
would appear to be the only options capable of achieving global temperature
stabilization, or reduction, on a time scale of a few years commensurate with the
uncertainties of predicting signiﬁcant transients. Two general arguments against
all SRM techniques are that they do not directly retard ocean acidiﬁcation, and
their regional impact is difﬁcult to assess with current climate models, but they
might at least buy time and allow the world to avoid some of the more extreme
temperature or precipitation scenarios.
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2. Particle choice and properties
The choice of particle is receiving close attention; hitherto, it had been assumed
that aerosols would be sulphuric acid mists similar to those produced by
volcanoes. Such natural mists are efﬁcient scatterers of visible light from the
Sun and very inefﬁcient scatterers of infrared radiation from the Earth, even
though they are slightly smaller than optimum [4]. However, it may be possible
to consider using other particles with better properties. The Royal Society report
on geoengineering comments [1]
Various other types of stratospheric aerosol particles have also been suggested (Teller
et al. 1997; Blackstock et al. 2009; Keith 2009; Katz 2009) [2,8–10]. Engineered aerosols
might enable scattering that did not produce so much diffuse illumination, potentially
circumventing a signiﬁcant side-effect of sulphate aerosols. Alternative materials might also
avoid the coagulation and vaporisation problems that will be signiﬁcant for sulphate aerosols.
Finally, it is possible that advanced engineered particles could be designed that had longer
lifetimes, or that were lofted out of the lower stratosphere, so reducing the impact of the
aerosol on ozone chemistry, or enabling radiative forcing to be concentrated in special
locations such as the polar regions.

If other particles are to be designed and manufactured for use in a suspended
pipe system, they will need particular properties to be attractive alternatives to
the use of a sulphuric acid aerosol. As well as having a high refractive index and
suitable particle size to maximize solar radiation scattering, they should, for a
given amount of scattering,
— minimize stratospheric ozone destruction by having a lower heterogeneous
reaction rate than sulphuric acid aerosols for the following reactions:
(i) 2O3 → 3O2
(ii) N2 O5 + H2 O → 2HNO3
(iii) HCl + ClONO2 → Cl2 (g) + HNO3
— minimize regional changes in precipitation by reducing stratosphere
heating through having a lower absorption of solar radiation than for
sulphuric acid aerosols.
The particle surface coating technology needs also to:
— Provide dispersion of the particles in the chosen carrier ﬂuid both in
the pipe at very high pressures (at up to 6000 bar), and at the point of
discharge under low pressure (less than 0.1 bar, −50◦ C). Electrical ﬁeld
gradients are likely to be signiﬁcant at the start of the plume and may be
used to reduce coagulation. The carrier ﬂuid composition must ensure an
unreactive, buoyant plume at 20 km altitude even with signiﬁcant particle
loading. Nitrogen might be a suitable candidate carrier gas, with added
hydrogen to ensure necessary buoyancy.
— Be stable for at least two years in the cold but high UV conditions of the
upper atmosphere.
— Be designed to minimize nucleation effects in the troposphere after the
particles have left the stratosphere.
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Manufacture of the particles should be low cost, with a low environmental impact,
and the particles must have negligible toxicity. Ideally, there should be a sufﬁcient
supply of the relevant raw materials to ensure availability for at least a century.
Various high refractive index particle systems could be considered but titanium
dioxide (TiO2 ) is a promising candidate. No other particle comes anywhere close
to its properties: it has a high refractive index, its safety has been well researched
and it is produced in industrial quantities. Coated TiO2 particles of a size
approximately 0.15–0.25 mm in diameter would be suitable. They are only slightly
smaller than those already produced for most paints and for providing opacity
in many other applications, e.g. in paper, plastic ﬁlms, inks, some foodstuffs.
A much smaller particle size (approx. 0.05 mm) is already made to scatter UV in
sunscreens. Titanium dioxide has the highest refractive index of any pigmentary
substance currently manufactured on a relevant scale which is stable in air and
non-toxic. It also has a low visible light absorption, and there is a vast experience
in manufacturing nanometre thickness coatings on titanium dioxide to control
surface properties [11,12].
The use of different coatings allows the possibility of minimizing atmospheric
chemistry effects. Current TiO2 particle sizes are optimized for light scattering
in close-packed systems, but the existing manufacturing units could easily be
modiﬁed to produce the slightly smaller sizes for light scattering in a dispersed
environment [11]. The inorganic/organic coating systems have the potential to
be modiﬁed to give stable hydrophobic surfaces with a variety of chemistries that
may reduce ozone destruction to a lower level than that of sulphuric acid mists.
Other potential high refractive index particle systems should also be considered.
By conveying the particles up the pipe in a supercritical ﬂuid, and making
use of existing compact and lightweight micronizing technologies employing the
conveying ﬂuid for motive power, there is the potential to create a well-dispersed
aerosol system at 20 km altitude.
Early experiments could be carried out from free-ﬂying balloons or fast jets,
possibly followed in due course with a high-pressure pipe supported from a
relatively small (approx. 100 m diameter) balloon. The latter system would be
able to create progressively larger plumes at altitude to enable the atmospheric
chemistry, dispersion scattering and electrical effects to be studied. Much of the
technology to examine such plumes is available from existing atmospheric science
studies using satellites, free-ﬂying balloons, aircraft and ground-based equipment.
Notwithstanding all of the above, the use of H2 S as a precursor system
for sulphuric acid aerosols has the great advantage of generating almost ﬁve
times the mass of the material lofted, through the supply of oxygen from
the stratosphere, i.e. H2 S + 2O2 → H2 SO4 . A particulate system also needs a
conveying gaseous material to be lofted. Scattering calculations and process
ﬂow analyses indicate that the conveying material mass ﬂow and the extra
mass provided by the ‘free oxygen’ are likely to negate mass reduction through
higher scattering per unit volume. However, creating an appropriate sulphuric
acid mist at altitude in a representative environment poses signiﬁcant problems.
Without that capability, the slowness of the reaction of SO2 to H2 SO4 means that
local, small-scale experiments would be impossible: atmospheric circulation would
disperse precursor material (SO2 or H2 S) around the planet before sulphuric acid
mists would form, and also before suitable measurements could be carried out on
their effects [4].
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3. Costing and development times of delivery technologies
A rational comparison of the technologies for delivery of aerosols into
the stratosphere must include outline estimates for the ﬁnancial costs
involved. Any such estimates will be based on various assumptions and
preconditions.
Assumptions
(i) Each technology requires at least four delivery sites around the globe in
non-polar regions, i.e. at latitudes within approximately 20◦ of the equator,
for effective dispersal of albedo-enhancing aerosols with a mass of around
10 million tonnes per year at above 20 km altitude.
To support this conjecture, it has been observed that volcanoes in the tropics
(e.g. Pinatubo, 1991, 15◦ N) have a greater effect on temperature than those
at higher latitudes (e.g. Katmai, 1912, 58◦ N). Preliminary modelling suggests
that this is because of a higher solar radiation ﬂux near the tropics and
stratospheric (Brewer–Dobson) circulation lofting particles injected near the
equator to high altitudes, whereas particles injected nearer the poles remain
close to the tropopause and are removed more rapidly [13]. Atmospheric currents
distribute the injected particles east–west within weeks, but more slowly (months)
north–south as shown, for example, in Pinatubo observations [14]. The amounts
of SO2 released by Pinatubo (18 ± 4 megatonnes SO2 equivalent) [15] can be
used to compare theory with actual events, and information is available on the
optical depth [16].
Pierce et al. give an overview of some of the uncertainties in estimating the
amount of material needed if SO2 is used as the aerosol precursor [17]. Some
models suggest that 2 million tonnes of sulphur per year, or 4 million tonnes of
SO2 per year, would be sufﬁcient to counter a doubling of CO2 levels by the use of
a smaller particle size than Pinatubo [18]; others say 10 million tonnes of SO2 per
year might be needed [19]. Estimates for the amount of material required vary
with particle size, refractive index, coagulation rate and stratosphere/troposphere
mixing rate. If the feed is SO2 , hydrolysis takes place to form a sulphuric acid
mist with a droplet size in the light-scattering range, with an e-folding time of
around 35 days in the relatively dry stratosphere [20], much as in the natural
processes of volcanoes with SO2 or H2 S, so dispersion is known to be relatively
straightforward. (H2 S oxidizes to SO2 and water relatively quickly, followed by
the slow reaction to H2 SO4 .)
The cost of producing the aerosol feed is assumed to be independent of the
delivery technology and is omitted from the comparisons, as have the costs of
basic infrastructure facilities for transporting materials to the launch sites and
intermediate storage. Personnel costs associated with running the various facilities
are generally ignored except when they become material to the comparisons,
although some personnel costs are implicitly included in the costs of the various
components. Costs quoted in references in US$ have been converted at the
rate $1.6 = £1. Signiﬁcant rounding has been applied given the tolerance of
the estimates.
(ii) Weather, particularly wind speed in the troposphere, has an appreciable
effect on most of the delivery systems under consideration. Lightning, icing
and turbulence can play a part in choosing the injection locations.
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The wind speed shown in ﬁgure 1 is taken from the maximum of the 6 hourly
‘instantaneous’ data for the wind (ERA-Interim) at 33◦ N [21]. The effect of the
jet stream at an elevation of about 11 km is clearly visible, as is the reduction
to relatively steady conditions at 20 km. The ﬁgures used reﬂect extremes in the
tropics if very dry (low storm intensity) regions are used for the injection points.
Surface drag reduces the wind speed at the Earth’s surface. For some calculations
the ﬁgures will need to be increased; very short gusts have structural effects, and
these ﬁgures do not include the effect of tropical storms that can cause very
large wind speeds at low levels. The peak wind speed at 20 km is only about
50 m s−1 , while the maximum peak wind speed measured in a year (1953) over
North America was 136 m s−1 [22]. While these data are relatively old, it remains
the most consistent dataset available for peak wind speeds and provides a useful
semi-continuous wind velocity function for optimizing engineering design.
The density variation (ﬁgure 2) is also important, falling to about a 14th of its
surface value at 20 km.
(iii) The systems considered below can be classiﬁed in various ways. Two
criteria are of particular importance: the length of time the delivery device
remains in the stratosphere and the reusability of the components.
It is assumed that:
— those systems where the delivery device follows a ballistic trajectory will
remain in the stratosphere for a very short time (of the order of seconds)
and will therefore require an extremely rapid dispersal process, either by
means of detonations or by using a system where the liquid or solid will
naturally disperse without mechanical assistance and
— those systems with a dwell time in the stratosphere measured in minutes
or hours, such as aircraft or cruise missile technology, could use some form
of mechanical dispersion.
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20

height above sea level (km)

16

12

8

4

0

0.5
air density

1.0

1.5

(kg m–3)

Figure 2. Variation of air density with height.

These short residence time systems are effectively batch processes. It is also
assumed that:
— those systems with a time in the stratosphere of many days or months,
such as tethered balloons or airships, could use quite sophisticated
dispersal technology and can be expected to give a much more
controlled dispersion. There will be a weight penalty since the machinery
would have to be carried to an altitude of 20 km, and powered continuously
at this height.
These long residence time systems are effectively continuous delivery processes.
(iv) The options for reusability raise different issues.
— For ‘one-shot’ systems, the multiple delivery elements have to be
cheap. The environmental cost of the redundant components must
be considered, both in terms of safety (heavy pieces falling back to
Earth or the possibility of noxious materials being strewn around) and
appearance (litter).
— Systems with limited reusability allow components to be recovered and
either used again immediately or refurbished. In this way, many of
the drawbacks of one-shot systems are removed. However, the costs
of recovery must be taken into account, or at least an allowance
made for the difﬁculty of making the system components return to
base autonomously. The cost of regularly providing replacements for
damaged components must also be considered.
— Systems with complete reusability. Systems that provide for continuous
use, or reuse over a long period, may justify high initial costs and
complexity.
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(v) The development times discussed assume a ‘Manhattan project approach’
where ‘political will’ ensures almost unlimited access to relevant resources:
people, ﬁnance, facilities. Such a position might develop when obvious
societal stress was arising from global warming, but would have greater
attendant risks.

(a) Free-ﬂying balloons
The concept of using cheap latex balloons, similar to weather balloons, to
lift a payload of particles has been considered by a number of authors [1,2,4].
The analysis given here closely follows that in COSEPUP [5] but uses different
values for some parameters. A 20 m diameter balloon has been assumed to be the
maximum practicable.
By application of Archimedes’ principle, a payload of around 250 kg can be
lofted to 20 km (where the density of air is around 0.08 kg m−3 ) by a single
hydrogen-ﬁlled balloon of 20 m in diameter. A latex or nylon balloon, typical of
those routinely used for scientiﬁc purposes, would weigh around 25 kg, and 20 kg
is allowed for a canister and valve needed to release the payload. It follows that
40 million balloon ﬂights per year are required to loft 10 million tonnes of aerosol.
The price of latex in July 2010 was £1.4 kg−1 [http://www3.lgm.gov.my/mre,
date accessed: 1 July 2010] and in July 2008 the hydrogen price stood at around
£1.6 kg−1 [http://www.hydrogencarsnow.com/blog2/index.php/hydrogen-fuelproduction/global-hydrogen-inc-lowers-price-of-hydrogen-to-247-per-gallon, date
accessed: 31 October 2010]. The cost for a canister and valve in bulk production
is estimated at £30.
The use of a premixed hydrogen/H2 S or SO2 system as the lifting gas has been
proposed [4]; this is entirely feasible but would increase the volume and therefore
the number of balloons by a factor of 5 with a consequential increase in cost.

(i) Single-use balloons
If individual balloons are not reused, one balloon is required per ﬂight. The
total cost of hydrogen is £1.4 billion, that of balloon fabric is £4 billion (with
fabrication costs on this scale being minimal when compared with material costs),
and that of canisters is £1.2 billion per year.
These numbers imply a total cost of approximately £7 billion per year, and
would be expected to increase in line with energy prices.
Global annual production of latex and hydrogen is 21 million tonnes
[http://www.lgm.gov.my/nrstat/T1.htm, date accessed: 31 October 2010] and 70
million tonnes [http://www.azom.com/news.asp?NewsID=20711, date accessed:
31 October 2010] respectively, so these requirements represent 11 and 1.4 per cent
of current global production. The largest delays in producing a single-use balloon
system are likely to be the difﬁculties of scaling up production and the provision
of ground facilities, so a system should be deployable within 5 years in extremis.
Latex is naturally biodegradable but the mechanism is not well understood [23].
It would need to be made more rapidly biodegradable, without affecting its
strength, otherwise the environmental impact of balloons littering the Earth’s
surface would be severe, particularly to marine life. The social impact would
be moderate, mostly affected by the extensive effect on air trafﬁc given the
unpredictability of the ﬂight path and the frequency of balloon launches. The
spent hydrogen would combine with oxygen in the atmosphere to release about
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10 million tonnes of water into the stratosphere each year; i.e. far less than
the 500 million tonnes threshold at which signiﬁcant effects on stratospheric
circulation might be expected [24].

(ii) Reusable balloons
Compared with the single-use balloon case, the number of balloons required
falls if they can be retrieved and reused. However, the cost and environmental
impact of retrieving the spent balloons is likely to be signiﬁcant, and could be
greater than for single-use balloons since greater care would be needed in retrieval
to ensure reuseability. Horizontal wind components of approximately 30–40 m s−1 ,
averaged over a 20 km column, are typical. If a balloon ﬂight lasts between 1 and
3 h, with a rate of ascent around 4–12 m s−1 , then the balloon will travel around
150–450 km, which if all directions are equally probable, equates to a retrieval
area of up to 600 000 km2 around each launch station. In practice, the area over
which the balloons would fall would be somewhat less, at least over short time
scales, since the wind direction is not random.
Environmental damage and wear-and-tear are likely to restrict the lifetime of
each balloon. A modern hot air balloon has a lifetime of approximately 700 ﬂight
hours. Time on the ground or in the sea would add to wear-and-tear. Keeping time
on the ground down to an average of 24 h, with 25 000 launches per day per site to
meet the four-site, 40 million balloon launches per year criterion, requires around
4000 retrieval teams per site if each can pick up six balloons per day and cover
approximately 10–75 km2 in unpopulated terrain such as the Australian outback.
If a larger number of sites were used, the number of teams would probably increase
since the balloons would typically be spread over a wider area.
A retrieval truck or boat with two operators might cost around £750 per
day including fuel and depreciation. This would amount to an expenditure of
16 000 teams × 365 × 750 ≈ £4.4 billion per annum (p.a.) plus launch costs for
the worst case and perhaps one quarter of this if high speed ascents and descents
are used. Assuming a round trip time from launch to launch of 3 days, at least
300 000 balloons would be needed. If the balloons last an average of 20 ﬂights,
then 1.3 million new balloons are needed each year, and if each reusable balloon
costs twice as much as a throw-away one, the cost would be about £400 million
p.a. It is assumed that the hydrogen cannot be reused.
The total cost is thus approximately £1–4.4 billion p.a. for recovery + £4 billion
p.a. hydrogen + £0.4 billion p.a. balloons, giving a total between £6 billion and
£9 billion p.a., which is at least as high as the cost of throw-away balloons, largely
because of the costs associated with recovery.
The technology is mostly available today, but there would probably need to be
a large investment in establishing the necessary supply routes to the launch sites,
which would probably mean a delay of 5–10 years before such a system could
be deployed effectively. Although the pollution associated with discarded balloon
fabric of one-shot balloons would not be incurred, the effects of the release of
large amounts of hydrogen at high altitude would remain. There would be the
same issues associated with interference with air trafﬁc, so social impacts would
be similar to single use balloons.
The idea of using superheated sulphur dioxide itself as both the buoyant gas
and the payload of the balloon was rejected on the grounds that it would require
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temperatures of around 600◦ C, well above the melting point of any low cost
balloon fabric.
When considering the dispersal of aerosols other than those derived from a
precursor gas, both balloon options become more expensive on account of the
dispersion equipment that has to be carried to high altitude.

(b) Towers
It is difﬁcult to conceive of any tower option being achievable in the
foreseeable future, but they have been proposed and should be considered. Both
environmental and social costs can perhaps be considered to be moderate rather
than high but would be overwhelmed by the effects of the massive ﬁnancial
investment needed for towers.

(i) Conventional towers
Tall towers have been mentioned as potential options for elevating and
dispersing material to stratospheric altitudes [4,25]; they would have the
advantage that they could operate continuously while providing human access
to the dispersal equipment, but the simple analyses described below show that
such a tower built with any materials currently available is not practicable and
would certainly be expensive (£250 billion in materials alone for each tower).
Some savings could be obtained if a 15 km tower (rather than a 20 km tower)
were built on a high plateau, for example, in the Andes or Himalayas.
A tall tower, to carry only its self-weight and wind loading, must satisfy two
basic criteria. It must be strong enough that the structural materials do not reach
their limiting stresses and it must be stiff enough that it will not buckle. In the
ﬁrst instance, these criteria can be considered separately and, for illustration,
the designs have been carried out in steel and in carbon ﬁbre reinforced polymer
(CFRP) with the material properties listed in table 1.

Strength design
A straight tower, of uniform cross section, will fail in compression when the
height is equal to s/rm g, where s is the strength of the material in compression,
rm the density and g the acceleration owing to gravity. This is a limiting factor
for steel, giving a maximum height (without any safety factor) of approximately
6.5 km. However, as will be seen below, this height can be exceeded if the
tower tapers.
The predominant wind forces will be horizontal, so the tower will act primarily
in ﬂexure as a vertical cantilever. Two tower cross sections are considered: one
formed from a single hollow tube as shown in ﬁgure 3a, rather like a continually
tapering cooling tower; the other from four legs with bracing, ﬁgure 3b, like
the Eiffel Tower or a UK electricity pylon. Most tower conﬁgurations can be
approximated to one or other of these cross sections.
The single tube will be thin walled and it is assumed that the wall thickness t
is R/50. Thin-walled tubes in compression are at risk from local buckling where
the thin wall crumples. For this failure mechanism to be avoided stiffeners in
the form of internal bracing will probably be required but it is assumed that the
stiffeners are light enough to be neglected in the calculation of tower mass. For
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the truss, a signiﬁcant amount of bracing would be required, as in an electricity
pylon. It will be assumed that this bracing weighs as much as the four main legs
and attracts the same amount of wind load. The diameter of each leg d is related
to the overall tower width D by taking D/d fairly arbitrarily as 25.
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The steady wind load on the tower is given by F = (1/2)ra CD v 2 A, where F
is the force, ra the air density, CD the drag coefﬁcient, A the area facing the
wind and v the wind speed. The wind speeds in ﬁgure 1 certainly underestimate
the maximum 3 s gust normally used for structural design but are used here in
the absence of better data. The drag coefﬁcient depends on Reynolds number
(Re), but for the structures and wind speeds being considered Re is greater than
8 × 104 so CD can conservatively be taken as 0.5 (e.g. [26]).
To determine the section dimensions the loads are integrated from the top of
the tower downwards, giving a vertical moment (from the wind) and an axial
force (from the self-weight). The top of the cylindrical tower is assumed to have
a radius R of 0.5 m and the truss a width D of 1 m. By computing the sum of
the axial stress and the bending stress and comparing this total with the limiting
material stress sy , the required diameter at each level can be found. No allowance
has been made for the deﬂection of the tower above the point in question, which
would increase the forces and hence increase the size of the tower cross section.
For the hollow tube design, the variation of R with height is shown in ﬁgure 4.
For steel, the radius at the base is about 68 m with a total weight of steel of about
19 million tonnes. Using a lighter material such as CFRP the width increases less
rapidly, thus reducing the wind load, so R at the base is only 26 m with a total
weight of 0.66 million tonnes.
Similar effects are observed for the trussed form, with the variations shown
in ﬁgure 5. The width of the steel truss increases quite rapidly towards the
base, indicating that the self-weight of the tower is starting to be a much more
signiﬁcant part of the load, although it is still possible to build a tower 20 km
tall. The trussed steel tower would have a total weight of 16 million tonnes, half
of which is assumed to be in some kind of bracing. There is much less weight in
the CFRP tower (218 000 tonnes).
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Stability design
These dimensions, although large, give structures that are very slender. Even
for the steel tube, the ratio of the height to the width at the base is about
150, which is comparable to a very slender ﬂag pole; such structures are very
susceptible to buckling under their own self-weight. In the ﬁrst instance, buckling
will be computed on the assumption that the towers are of uniform cross section
for their entire height. Later, tapered towers will be considered.
The well-known formula for Euler buckling load of a uniform pin-ended strut
under externally applied axial load is Pcr = p2 EI/L2 , where E is the Young’s
modulus of the strut material, I its second moment of area and L its length.
For a cantilever column, this buckling load must be divided by 4 to reﬂect the
different boundary conditions. When the buckling load comes from the tower’s
own self-weight, the classical analysis of a uniform cantilever column gives a value
for the critical load as
7.837EI
,
(3.1)
(qL)crit =
L2
where q is the weight per unit length [27]. The 7.837 factor can be compared with
p2 /4 = 2.467 for the end-loaded column; clearly, more load can be carried if it is
distributed evenly along the length rather than being carried at the end. But the
self-weight is related to the material density and its cross-sectional area so the
formula above can be rearranged to give

E r 2
,
(3.2)
Lcrit = 7.837
rm g
L
where r is the radius of gyration of the tower cross section. This formula
conveniently separates the buckling load into a material factor
√ and a shape
factor. For a thin circular tube, the radius of gyration r is R/ 2 (and notably
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is independent of the tube thickness). Even for a solid cylinder r only decreases
to R/2, so the values determined
√ below apply to a wide range of cross-section
options. For the truss r = D/(2 2), allowing for the fact that half of the material
is in the bracing that contributes to the weight but not to the overall stiffness of
the tower.
For steel, the required dimensions for the uniform tube at 6.5 km tall would
give r = 116 m whence R = 163 m for the tube or D = 327 m for the truss. For a
20 km tower in CFRP, the radius of the uniform tube is 500 m. These dimensions
are clearly much larger than the values given by the strength analysis, but do not
take account of the reduction in section at the top: these values do not include
any safety factor.
For a tapered tube there is typically no closed-form solution. An approximate
Rayleigh analysis for the self-weight buckling load can be performed by assuming
the shape of the buckling mode and equating the strain energy of ﬂexure to the
work done by the load. The exact analysis would predict a lower buckling load
because the assumed shape will need more strain energy than the correct form.
A commonly assumed buckling mode is to calculate the shape the tower would
adopt if it were mounted horizontally and subjected to a gravity load [27]. By this
approximation, the Rayleigh analysis for a uniform section (for which an exact
solution is available) overestimates the critical length by only 0.1 per cent, so it
is reasonable to use the same approximation for a tapered tower.
A Rayleigh analysis indicates that the tapered steel tube tower, with R = 68 m
at the base designed to resist the wind load in ﬂexure, and with a mode predicted
as above would buckle if the gravitational acceleration gcr were 0.166 m s−2 . Since
the buckling load will vary as R2 , in order to make the tube buckle when gcr is
9.81 m s−2 , the tube dimensions would have to be increased by a factor of about
7.6, giving a diameter at the base just over 1 km—which is a tube into which a
large football stadium would ﬁt, complete with its carpark.
Similar analyses have been carried out for tubes and trusses in both steel
and CFRP. The results are given in table 1. The ‘cheapest’ of these options
(shown in bold in the table) is the shaped cylindrical tower in CFRP, at about
£250 billion each. If four such towers are required, the cost would be of the
order of £1 trillion with 250 million tonnes of CFRP needed. No allowance has
been made for the difﬁculty of working at the altitudes needed. Figure 6 shows
these towers in comparison with some well-known landmarks to give some sense
of scale.
The choice of materials for illustration was determined by material properties:
since buckling is the governing condition, it is the speciﬁc modulus (E/r) that
matters. Steel is stiff but heavy, so E/r is low, although it has the advantage of
being plentiful and relatively cheap. Carbon ﬁbre has a similar modulus but is
lighter, so E/r is higher, but it is much more expensive. Most other structural
materials fall within this range; aluminium, for example, has both a modulus and
a density that are about one-third those of steel, so the same amount of material
would be needed, but at a much greater cost. The only way to make a signiﬁcant
improvement would be to increase E, but this is limited (at least for organic
materials) to the stiffness of the C–C bond, which is about 600 GPa, while at the
same time decreasing the density. Even if materials like graphene were available
in industrial quantities, they could only increase the speciﬁc modulus by at most
a factor of 2.
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shaped
CFRP tube

normal flight level

Everest

uniform
steel tube
Mont Blanc

Ben Nevis

CN KVLY
Tower Mast

Burj
Great Khalifa
Pyramid

Figure 6. Comparison with notable tall structures (to scale).

Cost
There would be major supply issues for carbon ﬁbre; current world production
is of the order of 50 000 tonnes p.a. [http://www.prnewswire.com/news-releases/
reportlinker-adds-global-and-china-carbon-ﬁber-industry-report-2010-101893378.
html, date accessed: 31 October 2010] with around $460 million for a 4800 tonne
p.a. marginal increase including impregnation facilities [http://pubs.acs.org/cen/
news/85/i08/8508notw2.html, date accessed: 31 October 2010]. At £60 000
per tonne p.a., a few trillion pounds would be needed to increase carbon ﬁbre
production to the 10+ million tonnes p.a. needed to make the materials for these
towers (approx. 100 million tonnes of carbon ﬁbre) over a 10 year period. Even if
economies of scale meant that the cost of providing new plant reduced by 20 per
cent each time production was doubled, scaling up production gives a cost (£1–2
trillion) that is comparable to the material cost quoted above, which neglects
erection and fabrication costs. This approach would clearly need a very long time
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to scale up production, followed by another very long time for the actual building;
50–100 years does not seem unreasonable.
Signiﬁcant savings could be achieved if the CFRP tower were founded on
a 5 km plateau; the base diameter would reduce to 350 m, and both the
material weight and its cost would reduce by 70 per cent to give a cost of £77
billion per tower. Access to infrastructure would have to be provided, but the
1000 km railway to Lhasa at 6000 m is reputed to have cost about $4 billion in
2006 [http://news.bbc.co.uk/1/hi/4345494.stm, date accessed: 31 October 2010].
Given the costs of a tower it would be worth constructing a road up Mt Everest
and building a tower there!
The analysis presented above is simple, but reasonable. A more extensive
variational analysis to choose the optimal shape of the tower, to resist both global
buckling and ﬂexural stresses, could be carried out, which might reduce the total
weight of the tower. However, allowance would have to be made for reductions
in stability caused by wind-induced deﬂections (a full wind load on the CFRP
tube gives a deﬂection at the top of about 0.35 km). The ability to resist local
buckling effects, distortions of the cross section and vibration of the tower when
subject to gusting winds, all of which have been ignored here, would add weight
to the tower. The foundations would need careful consideration, as would the
tower’s susceptibility to earthquake and even possibly local effects of applying a
very high point load to the Earth’s crust.

(ii) Guyed masts
Many of the existing studies of very tall towers, for example, Bolonkin [28], who
proposes towers 100 km tall, and others [29], avoid the problem of buckling by
assuming that the tower can be guyed. This is also the normal form of construction
for tall unoccupied structures such as transmitter towers. However, the guys at the
top of a 20 km tower would need to be about 30 km long. Depending on the degree
of sag the designer chooses to allow, the tension in a cable hanging as a catenary
between two supports, without any external load, can be many times its own
weight [30]. The analysis is highly nonlinear [31], so it is difﬁcult to come up with
a simple factor without deﬁning a very speciﬁc set of conditions, but if the ratio
of the cable tension to the cable weight were a factor of 10, which would not be
unreasonable, a material capable of supporting 300 km of itself in simple tension
would be required, even before taking into account a safety factor, difﬁculties of
anchorage, or creep. The only material commercially available at present would
be poly-p-phenylenebenzobisoxazole (PBO); graphene and nanotubes might be
possible if the strengths observed on minute quantities in laboratories could be
reproduced in bulk but this has yet to be demonstrated. There are other issues:
allowing more sag reduces the tension, but in turn reduces the effectiveness of
the guys and allows the mast more freedom to move. The vertical component
of the guy forces adds to the weight of the mast, thus exacerbating the very
problem they are designed to solve. Inclined stay cables in bridges are known
to be susceptible to wind-induced vibration [32] and are often themselves guyed
with counter-cables and provided with dampers.
The guys would impose very large lateral loads at the top of the tower; once
in place, the tower should be in equilibrium, but during erection it would not be
in balance unless very complex rigging schemes were devised. It is no coincidence
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that the world’s tallest radio mast collapsed while an individual guy was being
replaced [http://en.wikipedia.org/wiki/Warsaw_Radio_Mast, date accessed: 31
October 2010].
It is believed that the complexities of design, construction and maintenance
would make the erection of a guyed tower impossible.

(iii) Inﬂatable towers
The construction of towers from inﬂatable tubes made from lightweight
materials has been proposed [33]. That idea was used as the basis for a tower
designed to reach an altitude of 20 km from a mountain-top plateau at a height
of 5 km.
Unfortunately, no account is taken of sideways loads because of even moderate
winds, to say nothing of jet streams, and a cursory analysis shows such a
proposition is impracticable with any currently known construction materials.
In addition, there is an error in the original concept, which assumes that the
prestress from the internal pressure counteracts the structure’s own weight,
thereby preventing buckling, which is then ignored. But the prestress is part
of a set of purely internal, self-equilibrating, forces, and although they might
prevent local buckling of the skin they cannot prevent global buckling. Inﬂatable
towers would be subject to exactly the same buckling conditions as have been
described above. There is a direct analogy (although in reverse) with a bicycle
brake cable; no matter how hard the cyclist applies the brake, the cable does not
buckle because the compression in the cable sheath is equilibrated by an equal
and opposite tension in the brake cable.

(c) Aircraft
Systems based on aircraft have the advantage of using modiﬁed military
vehicles; they can loiter in the stratosphere long enough to disperse the particles
effectively and would be reusable. They have the disadvantage that there would
be a signiﬁcant energy cost in getting to the required height, and there would be
a signiﬁcant modiﬁcation cost. The analysis below is adapted from that given by
Robock et al. [4].

(i) Fast jets (F-15)
The F-15 has a maximum payload of around 10 tonnes [http://www.aerospace
web.org/aircraft/ﬁghter/f15, date accessed: 31 October 2010] so injecting 10
million tonnes of aerosols into the atmosphere requires around a million ﬂights
per year. If each plane is capable of performing three 2 h ﬂights per day, or
roughly 1000 ﬂights per year, a ﬂeet of 1000 planes should be able to deliver
the intended annual payload. The cost per plane is about £25 million. A ﬂeet
of 1000 modiﬁed planes would therefore incur a total capital cost of £25 billion.
As the F-15 has a lifetime of only around 16 000 ﬂight hours [http://www.fas.
org/programs/ssp/man/uswpns/air/ﬁghter/f15.html, date accessed: 31 October
2010] the planes would need to be replaced every 8 years, the annualized average
replacement cost would therefore be around £3 billion.
The operating costs are classiﬁed, but for tanker planes such as the KC-135 a
ﬁgure of $4.6 million is quoted for 435 ﬂight hours in 2003 dollars, which translates
to about £8000 per ﬂight hour today [http://www.gao.gov/new.items/d03938t.
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(ii) Tanker jets (KC-10 and KC-135)
The analysis is similar to the section above but the maximum altitude the
tanker jets can reach is below that needed to get above the tropopause except in
polar regions where injection is less effective. The payload of the KC-10 is around
80 tonnes [http://www.af.mil/information/factsheets/factsheet.asp?fsID=109,
date accessed: 31 October 2010], so with 1000 ﬂights p.a. a total of 125 planes
would be required. In 1998, the KC-10 unit cost was $88 million, corresponding
to a cost today of £70 million or £9 billion for a ﬂeet of 125. At 40 000 ﬂight
hours
[http://www.fas.org/programs/ssp/man/uswpns/air/ﬁghter/f15.html,
date accessed: 31 October 2010], the lifetimes are considerably longer than those
of the F-15 and thus a ﬂeet replacement would only be necessary every 20 years.
On average, therefore, replacement capital costs for planes amount to £440 million
p.a. For 125 planes operating costs are 125 × 2000 ﬂight hours × £8000 = £2
billion. The total cost for tanker jets would be £9 billion capital + £0.44 billion
replacement p.a. + £2 billion operating costs p.a.
Older KC-135 tankers have a lower payload of around 40 tonnes [http://www.
aerospaceweb.org/aircraft/transport-m/c135, date accessed: 31 October 2010],
requiring a ﬂeet of 250 planes. The unit cost is about £32 million leading to
a capital cost of £8 billion; if replaced every 20 years the annualized cost would
be £400 million. With twice as many planes the operating costs would be doubled
to about £4 billion p.a.
A hybrid system, in which the tanker carries both fuel and payload to 12 km
height, and then transfers both to an F-15 using air-to-air refuelling techniques
could also be considered but has not been costed.
The costs, particularly for the small jets, are related to high-performance
military units; it could be expected that in the next 20–30 years the capital costs
might reduce somewhat. It would also be possible to consider designing a new
plane tailored to the role of stratospheric particle injection [3]. However, the
operating costs would be likely to increase substantially in line with projected
increases in energy costs. It should be possible to deploy a system such as this
relatively quickly in an emergency, since a signiﬁcant number of military jets
are available now and would need relatively little modiﬁcation, but there would
almost certainly need to be new production if a permanent deployment were
required. However, the political will required would be extreme reﬂecting the
costs and environmental impacts: these are roughly equivalent to about 5 per cent
of the current world wide passenger air trafﬁc. It is presumed that the airbases
would be located away from populated areas, to minimize noise pollution, and
would be kept away from normal ﬂight routes to avoid air trafﬁc problems, giving
a high environmental but moderate social cost.

(d) Artillery
The largest naval artillery shell that could be ﬁred in the twentieth century
weighed 1510 kg using the 18 inch (0.457 m) barrel designed for HMS Furious in
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the First World War [34]. The size of the gun was effectively limited by the size of
the ship needed to support it; the barrel itself weighed 151 tonnes and had a very
complex form of construction. Such guns had a muzzle velocity of 738 m s−1 and
a horizontal range of about 37 km. Air drag (FD = (1/2)CD rv 2 A) is signiﬁcant;
equating the change in kinetic and potential energy with the frictional drag for
a shell moving vertically gives

g
1 CD A
dv
=
+
rv .
(3.3)
−
dh
v 2 m
The drag coefﬁcient for a streamlined projectile is reasonably constant in the
subsonic and trans-sonic regions at around CD = 0.5 and if it is assumed that air
density decreases linearly with altitude from 1.2 to 0.1 kg m−3 at 20 km, then a
simple numerical integration shows that if a 1510 kg shell were ﬁred vertically it
would need to have a muzzle velocity of about 800 m s−1 (Mach 2.6) just to reach
20 km. Smaller or heavier shells travel further.
It is assumed here that a similar gun, ﬁring a 1500 kg shell, of which 800 kg
is the aerosol payload, could be (re)developed relatively easily, and that it could
be mounted on land, or on a specialized barge. The shells would need to be
robust in order to withstand the accelerations when ﬁring, and they would have
to be intelligent to disperse the payload at the right time: requirements that might
be difﬁcult to achieve simultaneously. To lift 10 million tonnes p.a. would require
12.5 million shots a year. Inﬂation-adjusted estimates based on [5] for a slightly
smaller shell give a 2009 cost of £15 000 per shot, or about £200 billion for shells
per year. Gun barrels would need to be replaced at least every 3000 shots, because
of wear and the effects of the very hot gases, at a cost of £1 million per barrel,
and this rate of replacement assumes that relatively low accuracy is required
for stratospheric lofting. Total annual expenditure on barrels would therefore be
about £4.2 billion.
At a ﬁring rate of 5 shots per hour, with continuous ﬁring throughout the
year, some 290 barrels would be required at any one time, and each barrel
would need replacing every 25 days. There would be large setup costs but these
would be small by comparison with the annual cost of shells (£200 billion) and
barrels (£4 billion).
These cost estimates may be considered too high because no account has been
made that the shell price might be expected to reduce given the scale of the operation but this will be offset by the environmental costs associated with recovery
and clean up of the spent shells. There would probably be public opposition to
the establishment of such a large arsenal of ‘weapons’, and this option has been
classiﬁed with a moderate environmental and moderate social cost.
Although a system such as this would make use of 100-year-old technologies,
there remain difﬁculties of making large gun barrels to withstand the very high
pressures from the propellant, and it is probable that it would take many years
and a large investment in heavy manufacturing facilities before an artillery system
could be deployed.

(e) Missiles
Missiles differ from artillery in that they possess their own means of propulsion
and guidance. There is no longer a need for the gun barrels that made up a large
proportion of the cost of the artillery option.
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(i) Single-use missiles
‘Battleﬁeld range ballistic missiles’ (BRBMs) such as the Pakistani Hatf-1
have a range of 70 km, a weight of 1500 kg and a payload of 500 kg [http://
www.fas.org/nuke/guide/pakistan/missile/hatf-1.htm, date accessed: 31 October
2010]. If this could be modiﬁed without incurring too much expense to yield a
stratospheric rocket with a weight of approximately 2000 kg capable of lofting 1
tonne of aerosols into the stratosphere, delivering 10 million tonnes of aerosols per
year would require 10 million ﬂights per year. If particles are dispersed explosively
and missiles are not retrieved, then this is also the annual number of rockets
required. As a possible point of reference for the price, a sophisticated longrange cruise missile such as the Tomahawk has a unit cost of around £300 000
[http://www.fas.org/man/dod-101/sys/smart/bgm-109.htm, date accessed: 31
October 2010], thus one might assume that the short range, technologically less
complex missiles required for stratospheric lofting might be procurable at a far
reduced unit cost of £20 000. To inject the required amount of aerosol into the
stratosphere would therefore cost 2 × 104 × 107 = £200 billion annually.
The technology is available today, but the systems are complex, so it is assumed
that it would take several years before production could be increased sufﬁciently
for such a system to be deployed effectively. There would also be signiﬁcant
pollution from the exhaust gases, and expended missile casings. As with artillery,
it is to be expected that there would be public objections to the signiﬁcant
expansion in the production of war-like systems, resulting in a classiﬁcation of
high environmental and moderate social impact.

(ii) Retrievable missiles
The cost of missiles can be signiﬁcantly reduced if individual missiles are
reused, in which case they would have to make a controlled landing. They
would thus need to be powered after dispersing the payload so might be much
more closely based on cruise missiles or a new generation of high performance
UAVs. They would be expected to carry a more sophisticated particle dispersion
technology and a landing mechanism, potentially doubling the unit cost to
£40 000. In addition, individual missiles would need to be refuelled before each
ﬂight. If, as with balloons, one assumes 20 ﬂights a year for each missile, then
once retrieval and refuelling have been taken into account, about 500 000 missiles
would be required. The initial expenditure on missiles would be £20 billion, onetenth of the cost of one-shot missiles. However, the lifetime of the missiles can
be expected to be of the order of one year, so this is probably also the annual
replacement cost.
Although BRBMs generally use solid rocket fuel the price of jet fuel is
used here as a ﬁrst approximation. If 1000 kg of a missile weighing 2000 kg
is fuel, and the price of jet fuel is £0.42 kg−1 [http://www.iata.org/whatwedo/
economics/fuel_monitor/Pages/index.aspx, date accessed: 7 July 2010], then the
fuel cost for 10 million ﬂights per year is £4.2 billion. Total costs are thus of
the order of £25 billion p.a. As with single-use missiles, the technology largely
exists today, although the rate of production would need scaling up very rapidly,
and there would be the same concerns about exhaust gases. Overall, retrievable
missiles are classiﬁed with a lower environmental impact than single use missiles
with a comparable social impact.
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(f ) Electrical systems
Electrical technology has developed to the extent that it is now feasible to use
it to accelerate an object to the speeds needed by weapons. For stratospheric
injection, the projectile would not need to be steered, so the system could be
a ﬁxed-axis device mounted inside a shaft in the ground, either vertically or,
more probably, slightly inclined towards an area where the debris could safely
fall. The naval gun discussed above achieves a muzzle velocity of 800 m s−1 with
1800 g acceleration in an 18 m barrel, whereas if the system consisted of a 1 km
long tube in the ground, the acceleration would reduce to about 35 g, which
would then allow a much lighter casing to be used. Two technologies can be
considered—railguns and coilguns.

(i) Railguns
Railguns are under development for use as weapons [http://en.wikipedia.org/
wiki/Railgun, date accessed: 31 October 2010]. Two rails carry a large current
that passes through the projectile; the interaction between these currents
generates a propulsive force. In essence, it is a conventional electric motor with
a single winding. The requirement for electrical contact between the rails and
the projectile would lead to friction and wear, especially for systems such as
those envisaged here that will be ﬁred frequently. The very high current would
also cause heating within the projectile, which would almost certainly impose
constraints on the type of payload.
A railgun can provide energies of the order of 50 MJ and seems feasible for SRM
operations in the near future [http://www.defenseindustrydaily.com/bae-produ
cing-scaleddown-rail-gun-naval-weapon-01986, date accessed: 31 October 2010]
but the use of coilguns presents a more suitable alternative and for this reason
the railgun option has not been costed.

(ii) Coilguns
Coilguns use linear motor technology [http://en.wikipedia.org/wiki/Coilgun,
date accessed: 31 October 2010]. By surrounding a tube with a series of annular
coils that are switched on and off in sequence, a ferromagnetic projectile can
be accelerated. As for the railgun, lower accelerations are experienced if the
tube is very long. Long tubes mean that less steel is needed both to provide
electromagnetic coupling and to resist the inertial forces, but more material is
required for the tube and more coils are needed. One of the most signiﬁcant
design considerations for such a gun would be the difﬁculty of switching the coils
on and off fast enough.
If it is assumed that the projectiles are of a similar weight to the artillery shells
discussed above (1500 kg), but with only 250 kg of steel and 1250 kg of payload,
then 8 million shots will be needed each year or about 1000 shots an hour. Each
shell would contain less steel but, unlike conventional guns where the barrel is
riﬂed to make the shell spin to achieve stability, it is probable that the projectiles
would have to be provided with deployable ﬁns, adding to the cost. It is assumed
that each shell would be cheaper than an artillery shell at about £10 000, which
would give an annual cost of £80 billion. Without the need to constrain very hot
high-pressure gases, the lifetime of the tubes is expected to be much longer than
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that of conventional gun barrels. If each gun can be ﬁred once every 5 min, only
about 80 coilguns would be needed. If it is assumed that each tube is mounted
in a separate shaft drilled into the ground that costs £10 million to build, with
another £10 million for the electrical equipment, then the capital cost is under
£2 billion.
The kinetic energy of each 1500 kg shell at the muzzle velocity of 800 m s−1 is
480 MJ giving a total annual energy requirement of about 4 × 1015 J. As a rough
guideline, consumer electricity prices in July 2010 were around £0.1 per kW h or
£3 × 10−8 per J, giving total annual energy costs of £120 million. If the efﬁciency
of the coilgun is 20 per cent, this would entail annual costs of £0.6 billion, which
is small by comparison with the cost of the projectiles.
The overall costs are thus £2 billion setup costs plus £80 billion p.a., mainly for
the shells themselves. The use of ﬁns opens the possibility of making a reusable
system, since it is conceivable that the shells could be programmed to glide back
unpowered to a landing site downrange. This would signiﬁcantly add to the cost
and complexity of the system but would save on the number of shells that would
be needed. The recovery option has not been costed because of the large number
of unknown factors, but as with balloons it is expected that recovery will be
a substantial contribution to the overall cost. It is assumed here that it would
take about 10 years to develop a working system and to construct the required
infrastructure, and there would be pollution issues associated with the spent
casings falling back to Earth. These systems have been classiﬁed as having a
moderate environmental impact as a result, but a low social impact.

(g) High altitude airships
Airships have the advantage, over free-ﬂying balloons, of being powered and
able (weather permitting) to navigate back to given launch points. In 2006,
Lockheed Martin was granted a $150 million contract by the US Army to
construct high altitude airships capable of reaching the stratosphere for battleﬁeld surveillance purposes [http://www.defenseindustrydaily.com/lockheedwins-1492m-contract-for-high-altitude-airship-updated-01607, date accessed: 31
October 2010] but the project was delayed in 2008, before being re-launched
in 2010 [http://remixxworld.blogspot.com/2010/09/lockheed-martins-highaltitude-airship.html, date accessed: 31 October 2010]. These airships are
designed to loiter for long periods in the relatively low wind speeds at 20 km
(compared with the higher wind speeds at lower altitudes) rather than shuttling
up and down frequently. Basing costs on such a design probably gives a lower
bound on the cost of an aerosol deployment system. The prospective cost per unit
has been estimated at ‘tens of millions of dollars’, so £6 million is assumed here
[http://www.usatoday.com/tech/science/space/2005-07-05-air-force-balloons_x.
htm, date accessed: 31 October 2010].
The payload of each airship (150 m in length with a diameter of 46 m) is
only 2 tonnes, so to loft 10 million tonnes of aerosols requires 5 million ﬂights
each year. If each airship is capable of performing two ﬂights per day, including
ascent, dispersal, descent and refuelling, then 7000 vessels are required, amounting
to an initial £42 billion for airships. Very extensive ground handling facilities
would be required, for which a similar amount probably needs to be added.
Each airship carries around 135 000 m3 of helium (chosen for safety purposes) for
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buoyancy [http://www.globalsecurity.org/intell/systems/haa.htm, date accessed:
31 October 2010]. In order to return to the ground after dispersing 2 tonnes of
aerosol the airship will need to discard 2 tonnes worth of lift and this is most
easily achieved by dumping roughly 20 per cent of the helium in the airship. At a
crude helium price of £1500 per million m3 [http://www.blm.gov/pgdata/etc/
medialib/blm/nm/programs/0/helium_docs.Par.50876.File.dat/FY2011 Posted
Price ﬁnal.pdf, date accessed: 31 October 2010], £40 is required on each
balloon per ﬂight, amounting to £200 million for 5 million ﬂights. However,
there are signiﬁcant concerns about the rate at which helium supplies are
being depleted [35] and the cost is unlikely to remain so low for long. If
each airship is designed to remain in the air for a year that gives a lifetime
of approximately 9000 ﬂight hours. If, as with multiple balloons, the average
ﬂight time is around 3 h and two ﬂights are performed each day that gives
a lifetime of 3000 ﬂights a ship, or around 4 years. If the ﬂeet is renewed
at this frequency, the average annualized replacement cost would be about
£10 billion.
The total is therefore about £80 billion startup costs, plus about £11 billion
p.a. It is assumed that this project would take 10–20 years to reach practicality
as a delivery system, and the side effects would be relatively small, apart from
the signiﬁcant amount of helium jettisoned during descent, giving a low social
and environmental impact.

(h) Mixing aerosols into the fuel supplies of commercial ﬂights
It has been suggested that sulphur compounds could be added to
the fuel on commercial airliners to permit the deployment of albedoenhancing aerosols without needing to expend large amounts of additional
energy and resources on speciﬁcally constructed aerosol lofting mechanisms
[http://groups.google.com/ group/geoengineering/web/jet-fuel-additive, date
accessed: 31 October 2010]. Commercial airliners only reach the stratosphere
when ﬂying over the poles. Elsewhere, the tropopause is above normal
ﬂight levels.
No modiﬁcations to the aircraft would be necessary and aerosols could be dispersed as part of jet engine exhaust. The practicality of such a scheme hinges
on the amount of aerosol that could be delivered. There are four polar ﬂight
routes over the North Pole and none over the South Pole, accommodating about
10 000 cross-polar ﬂights per year [http://www.icao.int/icao/en/assembl/a36/
wp/wp144_en.pdf, date accessed: 31 October 2010; http://travel.usatoday.
com/ﬂights/legacy/item.aspx?ak=68495594.blog&amp;type=blog, date accessed: 31
October 2010]. Most of these ﬂights operate between the eastern United States
and eastern Asia, corresponding to a distance of roughly 10 000 km. A Boeing 747
consumes some 17.5 l of fuel per km [http://www-personal.umich.edu/∼murty/
planetravel2/planetravel2.html, date accessed: 31 October 2010] corresponding
to 175 000 l or 140 tonnes. Assuming that 5 per cent of this fuel payload could
be allocated to transporting SO2 aerosols then this accounts for only about
70 000 tonnes of SO2 per year, or around 0.7 per cent of the necessary 10
million tonnes required. In addition, as discussed earlier, aerosols injected at
the poles are believed to be much less effective for SRM than aerosol injection
at mid-latitudes.
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(i) Balloon-supported high-pressure pipes
Pumping precursors to aerosols such as H2 S or SO2 via a pipe elevated by a
balloon or aerostat or has been suggested by a number of authors [36].
The concept that is described here was developed in 2009 by one of us and
has been reﬁned with the help of the co-authors: a large high-altitude balloon or
aerostat located at around 20 km altitude of sufﬁcient size can provide enough lift
to support its own weight as well as the weight of a ﬁbre-reinforced pipe, lifting
devices intermittently spaced along the tether, and the weight of the ﬂuid being
pumped through the pipe [6] (ﬁgure 7).
The balloon system has a low cost and only moderate difﬁculty of manufacture,
provided structural and stability considerations are satisﬁed. Some degree of
streamlining can also be considered, but this is outside the scope of the
current work.
However, the pipe needs considerable additional lift from aerodynamic surfaces
providing a high lift to drag ratio. These need to be attached at a variety of
altitudes to prevent the pipe from having too great an inclination to the vertical
when exposed to jet streams and also to ensure suitable launch and recovery
trajectories (see the forthcoming sections).
The analysis below is similar to that of Badesha et al. [37], where the wind
proﬁle was shown to be the most signiﬁcant design driver for both the balloon
size and tether tensions, and hence cost. Others also mention wind but do not
factor its signiﬁcant effect in their detailed analysis [3].
A design altitude of around 20 km was chosen to be just within the stratosphere,
above the tropopause, in near-equatorial regions, allowing the majority of the
material injected to circulate within the stratosphere and not immediately be lost
to the troposphere. A higher altitude might be preferable to reduce losses further
but a far larger balloon would be required to provide the necessary lift. The other
great advantage of the 20 km altitude is that the wind strengths are at their
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lowest at this altitude, typically being of the order of 20 m s−1 or less (see ﬁgure
1). However, the balloon needs to tolerate winds whose mean velocity is as much
as 45 m s−1 at this altitude (P. Davidson 2010, personal communication). The
wind speeds in ﬁgure 1 are taken from the ERA database and do not reﬂect short
duration (2 min or more) gust speeds. These must be taken into account to prevent
‘blow-over’: in this scenario, the wind load on tether and balloon is sufﬁcient to
cause the balloon to be dragged sideways and downwards, leading to a pronounced
fall in altitude of the balloon. This can drag the balloon into the higher wind
speeds encountered at lower altitudes resulting in balloon failure. A wind speed
of 55 m s−1 (continuous + 20%) has been used for ‘semi-static’ design purposes to
prevent blow-over. For blow-over calculations, only winds of duration of at least
2 min need be considered to generate sufﬁcient changes of height. Two minutes at
approximately 45 m s−1 implies a vertical or horizontal eddy size of at least 5 km,
which is unlikely at an altitude of 20 km, if the balloon is sited away from large
mountain ranges.
Still shorter time-constant transient gusts must be considered in analyses of the
dynamics of the system, and when ensuring the structural integrity of the tether
and the balloon. Analyses for balloons at 6 km altitude suggest that stress peaks
show relatively modest transients [38,39]. Extension of this analysis to 20 km will
require some additional turbulence data for suitable sites. Predictions have been
made about the response to severe turbulence of a smaller diameter tethered
balloon at 20 km altitude for a thunderstorm propagating across the system with
a core updraft and microburst. A benign balloon temporal response is reported,
with the tether length scale (diameter) reacting to a comparatively narrow part
of the turbulence energy spectrum that has very little energy. The model used
101 nodes to represent the balloon and tether, and a time duration of 2000 s with
a time step of 0.01 s [40].
Balloon launch and recovery present particular problems for a tethered balloon.
Free-ﬂying balloons move with the wind in ascent or descent and are only affected
by variations with a length scale similar to the balloon diameter, but a tethered
balloon is subject to cross winds (with a maximum dynamic load at around 10 km
altitude), so the balloon surface must be under tension from the launch level to
operating altitude, to avoid the possibility of waves developing on its surface that
could tear the fabric.
A conventional ballonet arrangement allows an internal air-ﬁlled balloon to
be deﬂated on ascent and inﬂated by fans (which must be powered) on descent
to accommodate a factor of 14 volume change in the lifting gas between 20 km
altitude and ground level. This restricts the maximum rate of descent because
of limitations on fan size and power. A similar problem has been described in
trajectory simulation of the descent of a tethered balloon [41].
Another issue is that of vortex-induced oscillations of tethered balloons in
directions both inline and transverse to the ﬂow [38]. The drag on the balloon is
less signiﬁcant than the tether drag, and furthermore active oscillation control
may be possible both with control surfaces on the tether and adjacent to
the balloon.
Twisting and kinking of the tether have been raised as potential issues with
variable wind direction and speed over the height of the tether. For kinking
to occur, the longitudinal tension in the tether would need to fall transiently
to zero which is not what is observed in the analyses described above. Three
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thousand barrage balloons in the Second World War were successfully deployed
in the most turbulent part of the lower atmosphere over the UK alone. This gives
encouragement that such issues will not give material problems.
The particle injection does not need to be continuous, allowing operation for
between 200 and 300 days of the year, given that the time constants of the
upper atmosphere are of the order of 1–2 years (as seen with the Mt Pinatubo
eruption). Jet streams occur at an altitude of around 10 (±3) km and the design
accommodates a peak jet stream velocity of around 95 m s−1 with 55 m s−1 winds
at 20 km in the same direction. The maximum pipe angle under these conditions
is between 10◦ and 35◦ to the vertical, so the tether length needs to be around
21.5 km. A large tethered balloon would best be deployed from a ship or an
island, or possibly in desert regions with low population densities but with good
rail transport links.
Siting considerations (leaving aside political questions) should preferably be
in equatorial regions, with dry tropospheric conditions for at least six months of
the year, leading to low lightning frequencies and minimal icing potential. Tether
surfaces that are hydrophobic may also help in this regard. The vertical altitude
over which ice can build up if suitable locations are chosen is likely to be small
(approx. 2 km) compared with the tether length (approx. 20 km), with typical
lapse rates of 6 K per thousand metres.
The tether acts both as a tension element and as a pipe carrying liquid at very
high pressures. It thus has to withstand very high longitudinal tensile stresses and
very high hoop stresses. Aramid ﬁbres appear to be the most probable candidate
materials from which to fabricate the tether. They are made on an industrial scale
by a number of manufacturers; world production is of the order of 30 000 tonnes
annually. They have a short-term strength of about 2700 MPa, and a density
of 1440 kg m−3 , giving a free length (the length of itself that it will support)
of about 190 km, which at ﬁrst sight appears more than adequate. However,
aramid ﬁbres are susceptible to creep rupture, which is a thermally activated
process [42] and allowance must be made for a 60 per cent ﬁll factor, the weight
of the product being delivered, ﬁbres to resist the high hydrostatic pressures in the
pipe, possible temperature effects from the product and from the environment,
the need to anchor the tether, and a safety factor. A design stress of 750 MPa
has been used here [42]. Although aramids are suited to this application, their
capabilities will be pushed to the limit. A potentially much stronger alternative
is PBO [43] with an allowable design stress of around 1500 MPa, and it has been
successfully anchored, in the same way as aramids, without the use of resin. Such
a doubling of design stress would allow the balloon volume to be halved. However,
there are no published data on its creep and creep-rupture properties, which are
currently being studied. Neither aramids nor PBO can carry axial compression
since they form kink bands (essentially buckles at the sub-ﬁlament scale), which
is another reason why the tether should be designed to be under tension at
all times.
Alternative materials have been considered. Carbon ﬁbres have strength
comparable to that of aramids, and can be stiffer, but they are always used
with resin (thus adding about 30% to the weight of the tether) and are difﬁcult
to anchor [44]. Their use would also make the tether electrically conducting. A
lightning strike passing down the tether might destroy it. Another alternative
would be ultra high modulus polyethylene (UHMPE) ﬁbres, which are widely
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used in the marine industries for ropes and cables, and have the advantage of
being cheap. But they have the disadvantage that they creep which discounts
their use in a pressure pipe.
The design pressure of 6000 bar allows all pumping requirements to be satisﬁed
at ground level. The pumping power is determined by the hydrostatic head
(of the order of 3000 bar) and additional frictional pressure drop, which for
a low total system cost (pump + pipe + balloon) design is comparable to but
lower than the hydrostatic head. With a mean supercritical SO2 density of
around 1400 kg m−3 throughout the pipe, and a maximum velocity inside the
pipe of under 9 m s−1 , the frictional pressure drop is in the region of 2000 bar,
leaving a maximum total operating pressure at the pipe base of less than
6000 bar. Water-jet pumps at 6000 bar, with a capacity of around 7 l min−1 ,
have a cost of around £70 000 (manufacturer’s quotation, 2011), compared with
the required ﬂow of the order of 4000 l min−1 , but a scale-up in ﬂow rate of
the order of 10–100 seems entirely practical even if this requires a number of
parallel pumps. If high pressure positive displacement pumps have a cost versus
capacity exponent of around 0.65, 25 pumps with a capacity of 160 l min−1 will
cost around £630 000 each, and the whole pump assembly will cost around £16
million. Tripling this cost to allow for different materials and ground level pipe
work gives a cost per installation of around £50 million or £200 million for the
four installations.
As a cross-check, these numbers were compared with American data. The
capital cost for a 4000 bar, 100 hp (75 kW) water-jet pump with all infrastructure
is $62 500 [http://news.directindustry.com/press/jet-edge/jet-edge-introduceslow-cost-100hp-waterjet-pump-11866-333796.html, date accessed: 31 October
2010]. Scaling up using a 0.65 power rule gives the cost of a 6 MW pump as
(6/0.074)0.65 × $62 500 = $1 million each. Ten of these would be needed in each
location. So, for four locations, the cost would be $40 million. Allowing for more
sophisticated technology than simple water pumps might cost 200 per cent more,
suggesting a total capital of £75 million for the pumps for all four locations.
The energy required to lift the materials is only greater than the
thermodynamic minimum to the extent that the design incurs frictional
pressure drop, pump inefﬁciencies and dispersion power requirements. It is
thus unlikely to be bettered by any design that involves high-speed delivery
(artillery, rockets, etc.) or the use of intermittent carriers such as weather
balloons or jet aircraft, where the additional payloads to be carried to altitude,
or the energy costs of throwing away the carrier gases (either H2 or He)
are relatively prohibitive. For a pipe the pumping power is given by 500 ×
106 (Pa) × (100 (kg s−1 )/1400 (kg m−3 ))/0.6 (60% efﬁciency) = 60 MW. Assuming
power costs of £0.1 kW h−1 and with an 85 per cent motor efﬁciency, the cost is
about £7000 h−1 .
A tether of outer diameter (o.d.) 200 mm with an inner diameter (i.d.) of
100 mm would weigh around 800 tonnes (with a composite density of 1600 kg m−3
and with an additional weight of ﬂuid of around 250 tonnes). The sales price of
aramid ﬁbres is currently in the range $20–$30 kg−1 , so assuming a fabricated
cost of around $40 kg−1 and a tether length of 21.5 km the cost of each tether
would be in the range of £18 million.
Lifting devices comparable to small gliders attached to the tether mostly at
between 7 and 13 km in altitude dramatically reduce the size of the tether and
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balloon system. Approximately 4200 m2 of wing area is needed with an associated
cost (based on quoted prices for small commercial gliders) of £9 million.
Previous studies have considered smaller pipes for a ‘slurry system’ of
40 mm o.d. and 20 mm i.d. (for liquid), or 210 mm o.d. and 200 mm i.d. (for
gas) [3]. The cost comparisons are very different from those calculated here.
A material cost for pipes is given as $2.5 million, which seems reasonable,
but a development cost of $10 billion, just for the pipes, is used in their
cost estimates which seems excessively high. Those ﬁgures were based on
the assumption that a balloon system would be as complex as a deep-water
offshore oil production platform, which seems unreasonable. Many different
manufacturers are already making pipes of 5–20 cm diameter for use as undersea
ﬁbre optic cable runs, umbilical systems for offshore oil and gas production
or distributed district heating systems. Individual pipe runs can be many
kilometres long. Based on manufacturers’ quotations to the authors, and taking
a conservative estimate that the facility costs are proportional to the volume
of pipe produced, suggests a plant cost of about £40 million. The marginal
material cost of the tethers would be around £10 million each; assuming
seven tethers would be needed for development, with a development personnel
cost of 20 staff for ﬁve years at an all-in cost of £200 000 per person year
including equipment and facilities (£20 million) would make a total of £130
million. Allowing development and facility costs of the order of £130 million to
£250 million would seem to be adequate.
The balloon has to withstand gusts and is envisaged to be a pressurized
balloon with an operating differential pressure of around 800 Pa across the balloon
envelope. Operating with pumpkin balloons might allow less fabric weight but
such balloons have had launch issues, with large diameter balloons buckling at
intermediate altitudes [45]. A 375 mm balloon fabric wall thickness gives a low
wall stress of 170 N m−2 with a balloon weight of 160 tonnes and a payload of 10
tonnes (in addition to the weight of the tether).
One of the most contentious aspects of this proposition is the development and
costing of the very large balloon. McClellan et al. [3] make a comparison with an
airship and arrive at a costing of around $400 million for a volume equivalent to
that of a 300 m diameter balloon. For four units using these costs there would be
a capital outlay of $1.6 billion or £1 billion. However, an alternative strategy is to
manufacture cheap balloons and not to re-use them after each deployment. That
would obviate the need for expensive fan systems but could mean that descent
would be uncontrolled. Simple high altitude balloons can be made in relatively
cheap plant, essentially consisting of gore welding equipment, with the materials
costs being simply those of the balloon fabric and the gondola.
Estimates of costs have been provided by a commercial balloon manufacturer.
Fabric costs for material with a six month life at 20 km and design stresses
of 1500 N mm−1 are around £20 m−2 . Manufacturing costs are comparable with
fabric costs. Adding a 50 per cent margin gives conservative manufactured costs of
£60 m−2 . For a 315 m diameter balloon with a surface area of around 300 000 m2 ,
the manufactured cost on this basis would be around £18 million. A comparable
cost needs to be added for the ballonet, which has a similar amount of fabric,
giving a total balloon cost of around £40 million.
Alongside airships, this option has been classiﬁed as having a low
environmental and social impact.
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(i) Cost summary (excluding costs of aerosol and dispersal system)
The estimated costs for the balloon system are shown in table 2. It is assumed
that each balloon will operate for 7000 hours per year, giving an electrical power
cost of about £50 million per year per balloon. It is assumed that the balloon
and tether will be replaced each year and that annual pump maintenance will
cost 15 per cent of the pump capital cost, and the cost of providing the ships and
maintaining the launch sites will be £100 million per year. Together these give a
total operating cost of about £600 million per year.
Development costs are likely to be less than four years’ costs of a single
station if infrastructure costs (ships) and SO2 or particle generation facilities
are excluded, leaving a total for capital + development costs of £600 million.
Extended discussion on siting and legal/governance issues might increase these
to £700 million but this may be a signiﬁcant overestimate given the inherent
simplicity of the design. Allowing between £700 million and £1000 million in
development costs would seem to be adequate.
Such a sum would buy the plant to produce the tethers (£40 million), seven
tethers (at £10 million each), seven assemblies of lifting devices (at £9 million
each), seven large balloons (at £40 million each), a development, manufacturing,
legal and management staff of 400 for 4 years (£320 million), and site facilities
of £50 million.
If it were decided by a body, such as the Security Council of the UN, that
SRM had to be introduced as a matter of extreme urgency, it is believed that
this could be done within a period of 5 years and is the basis of the data point
plotted in ﬁgure 8.
The facilities for manufacture of the tether already exist in the offshore
industry, although some retooling would be necessary. Manufacturing facilities
for balloons larger than any built so far would need to be established, and
the balloons themselves tested, but this should just be a scale-up of existing
technology. Parallel developments would need to be carried out for pumping and
particle dispersal.
A more measured approach would however be highly desirable and might be
spread over a generation. It would allow reﬁnements of climate modelling, testing
of the albedo effects and of the atmospheric chemistry impacts of the particles.
It would allow testing of the actual delivery systems, ﬁrst at low level, then at
20 km: in the ﬁrst instance, nitrogen could be injected, followed later by injection
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Figure 8. Costs and development times (both on logarithmic scales) for various options, excluding
cost of material to be dispersed. Costs are net present costs taking a 5% discount rate and
amortizing over 10 years for the injection of 10 million tonnes p.a. into the stratosphere, to mitigate
the temperature effects of a doubling of CO2 levels.

of very limited amounts of particulate matter whose effects could be monitored
by ground-based, aerial or satellite observation. Only if these tests worked, and
then only if a supranational body such as the UN mandated the work, would
prototype and production versions of the system be implemented.
Were it found necessary to implement SRM climate remediation then the
authors estimate that the research, development and governance process may
take about 20 years with perhaps another 10–15 years for implementation in
normal conditions. The basis for this estimate along with estimates of costs for
each stage of the process are given in appendix A. These estimates are necessarily
very approximate given the uncertainties of future global climate, economics
and politics.
Appendix A shows an outline science and technology roadmap for developing
not only the delivery technology but also the particle science and technology,
the modelling of effect and some broad brush dialogue, communication, and
governance issues. It should be noted that the cost estimates are dominated by
the particle material costs if a manufactured particle (such as titanium dioxide)
is used. These would be the same for all systems.
This delivery method has four main developmental issues:
— the size of the balloon (signiﬁcantly larger than the world’s largest balloon
to date, which had a diameter of 120 m),
— the manufacture of a reliable high pressure tether,
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— the need to ensure that no transient oscillations or dynamics compromise
the integrity of the system, and
— the need to scale up pumping technology.
It is believed, however, that all of these factors, although challenging, are at the
edge of existing technology in one or more ﬁelds and can be overcome with suitable
development resource. Consideration is being given to the possibility of using
an aerodynamically shaped tether. Allowing for imperfections of manufacture,
the drag coefﬁcient for an aerodynamic tether would typically be a third to a
ﬁfth of that for a circular tether. Since the design case for the balloon is that
it must generate enough lift to prevent blow-over by the wind forces on the
tether, an aerodynamic tether with intermittent lifting surfaces has the potential
to signiﬁcantly reduce the size of the balloon by a factor of 2–3 in diameter.
However, each balloon would then only support a smaller pipe so more balloons
(by a factor of 15–20 times) would be needed. Such a design would allow smaller
scale tests to be carried out more readily. A stratospheric injection system with
eighty 125 m diameter balloons rather than four 315 m balloons could be envisaged
for a full-scale system. The total tether and balloon fabric weights for the two
systems would be comparable.

4. Comparing aerosol delivery technologies
It is now possible to compare the various options for delivering aerosols to the
stratosphere. Table 3 shows a comparison of the systems discussed above in purely
ﬁnancial terms; the costs are taken from the discussion made earlier and then a net
present cost has been calculated for establishing each technology and operating
it for 10 years, taking a discount rate of 5 per cent. Allowing for differences in
the injection rate, the costs show good agreement with McClellan et al. [3] for
the more limited number of technologies considered in that work, except for the
tethered balloon. This difference can be explained by the much larger development
costs suggested there, and the greater number of balloon systems.
Table 4 shows non-ﬁnancial aspects, including the time needed to set up
the facility.
Figure 8 shows the time and cost data together.
Rigid towers are extremely expensive mainly because of their very high initial
cost, although their running costs would be low. And uniquely among the
options they could allow manned access to a permanent dispersal facility at
high altitude.
Single-use missiles, artillery and coilguns are all expensive, primarily because of
the cost of the expendable delivery systems. It might well be possible, especially
for coilguns where the accelerations during launch can be kept fairly modest, for
the costs to be brought down if the delivery systems could be made to return
autonomously to a landing facility.
Most of the other systems cluster around the middle of the ﬁgure, with the
exception of the tethered balloon concept. It is worth reﬂecting on why
this system is so cheap to ensure that no important factor is missing from
the analysis.
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Table 3. Summary of delivery technology costs. The initial procurement cost refers to the purchase
price of capital necessary to begin delivery. In the case of technologies that involve ﬂeets of delivery
vehicles (e.g. planes, retrievable balloons, retrievable missiles) the cost of purchasing the initial ﬂeet
is taken. Single-use vehicles have procurement costs factored into operating cost. The net present
cost is computed assuming a discount rate of 5% and covering a period of 10 years.

— The connection between the ground and the stratosphere, although
permanent, is in tension and thus will tend to straighten, whereas the
tower is in compression and tends to buckle. The lift for the supporting
structure comes free, from the natural buoyancy of helium or hydrogen.
— Only the material to be dispensed at high altitude has to be lifted, and this
is done by pumps at ground level. There are no casings to be manufactured,
lifted, discarded or recovered.
— The accelerations during launch, pumping and recovery are all small, which
means that the system can be made from lightweight materials.
All of these factors, taken together, mean that the system has the potential to
have very much lower operating costs than the alternatives.
There is an additional advantage in that the system would remain in place
more or less continuously. Unlike systems that have to dispense their payload
in a very short time, the dispensing system can operate semi-continuously, and
would not get thrown away after every shot. This opens the way to improving
both the effectiveness of the dispersion techniques, and also the choice of particles
to be dispensed.

5. Conclusion
There may be arguments against SRM of any kind, for instance that it does
not directly retard ocean acidiﬁcation, and there may be arguments against
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Table 4. Estimates of non-ﬁnancial criteria. It should be noted that this is a summary for
comparison purposes only.

geoengineering itself. But while it is desirable that we work on reducing carbon
emissions now, it would be prudent to have emergency systems in reserve as an
insurance policy. We should design the emergency mechanism before we need it,
so that it can be tested to make sure that it is safe to use.
After considering the various options for SRM by stratospheric particle
injection, we suggest that a tethered balloon supporting a pressurized pipe is
likely to be efﬁcient, practical, controllable and much cheaper than any probable
alternative.
A tethered balloon system might be used to deliver SRM, not just as an
emergency measure, but also as part of a well-moderated and thoughtful process
of climate control.
Peter Davidson is employed by Davidson Technology Limited which has a patent application
pending regarding SRM technologies. Hugh Hunt and Chris Burgoyne are employed by the
University of Cambridge. They provided consultancy services for Davidson Technology Limited
in 2009, in their personal capacities, and were named as inventors on the patent application.
All ownership and rights in the patent application reside with Davidson Technology Limited.
Hugh Hunt and Chris Burgoyne are currently working on the SPICE project at the University
of Cambridge which is funded by EPSRC and NERC. Matt Causier is a research student at the
University of Cambridge also working on the SPICE project.
The work described here was partly funded by the Engineering and Physical Sciences Research
Council, the Natural Environment Research Council and Davidson Technology. The authors would
like to thank Tony Cox and Francis Pope (University of Cambridge), who provided input on the
chemistry of the stratosphere, Don Grainger and Dan Peters (Universities of Oxford and Bristol),
and John Temperley (Huntsman Tioxide) who provided input on scattering. In addition, Peter
Braesicke and Richard McMahon (University of Cambridge), Matt Watson (University of Bristol),
Olivier Boucher and Jim Haywood (Met Ofﬁce) and Lesley Gray (University of Oxford) provided
wind data and helpful comments on early versions of this paper. Hilary Costello and Kirsty Kuo
(University of Cambridge) have also provided background work on balloon and tether dynamics,
and with Jonathan Cooper (University of Liverpool) measurements of aerodynamic tether drag.
David Loew (University of Cambridge) carried out calculations on some of the comparative systems.
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Appendix A. An outline science and technology roadmap for SRM
climate remediation
1. Laboratory and concept development 3 years £2–3 million. Modelling of
climate at approximately 50 km horizontal cell size.
(a) Explore the potential feasibility of managing ozone destruction
processes in laboratory experiments, particularly by keeping N2 O5 and
chlorate surface reactions to a minimum.
(b) Demonstrate coating technologies in the laboratory that allow the
potential to achieve coatings that are stable for 3 years, that are
non-toxic and allow for 6000 bar slurrying and dispersion.
(c) Demonstrate light scattering in the laboratory by suitably dispersed
particles.
(d) Produce preliminary models of the impact on the ﬂora and fauna of
more diffuse light.
(e) Develop lower cost delivery technology: aerodynamic tether, vibration
model, pumping concepts.
(f) Carry out public engagement at a modest level:
(i) communicate and discuss the scale of the problem,
(ii) discuss the concept of research to minimize the risk that we may
have to carry out a ‘panic implementation’,
(iii) discuss the need to have a reversible intervention, and
(iv) discuss the strengths and weaknesses of the moral hazard
argument.
(g) Examine the advantages and drawbacks of pre-made particulates in
comparison with those created by slow hydrolysis of sulphates.
(h) Consider mechanisms that allow incremental testing of effect rather
than a step change approach.
(i) Initiate a scientiﬁc, engineering and social debate on modelling
needs and pace of implementation with a wide variety of
stakeholders. Encourage dialogue with governments and nongovernmental organizations including environmental groups.
(j) Develop scale-up and testing scenarios, and initiate discussion on
what tests need international agreement, e.g. would tests not
resulting in measureable ground illumination or precipitation changes
over inhabited areas or areas of special scientiﬁc interest be
routinely accepted?
2. Pre-trial development: injection of 100 tonne p.a. nitrogen at 20 km
altitude: approximately 3 years £6–10 million.
(a) Test pumping concepts, stabilization of tether, aerostat, launch and
recovery steps.
(b) Develop modelling scenarios: computing power, measurement and
science fundamentals.
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(c) Continue laboratory work on phase 1 science + measurement
techniques for operation off 20 km platforms and identify other
monitoring technologies suitable for observing the microphysics
of plumes.
(d) Begin dialogue with UN to establish mandate for micro- and small-scale
tests, governance and legal structure.
(e) Obtain buy-in to research from some mainstream environmental
organizations: consider options to protect biodiversity.
3. Micro trials at 20 km of 0.01 per cent full scale: 100 tonne p.a. or 2 tonne
per week approximately 3 years £40–80 million, approximately £10–20
million p.a.
(a) Develop mandate for small-scale tests.
(b) Establish a climate remediation R&amp;D centre on an ocean island with
UN support, relatively near the equator.
(c) Test delivery technology, dispersion and pumping technology at a
modest scale using a 150 000 m3 aerostat.
(d) Check atmospheric chemistry and opacity of plume, approximately
50 miles downstream, with balloons, satellites and aircraft; continue
laboratory R&amp;D and development work.
(e) Check coating stability, local weather effects.
(f) Demonstrate a variety of observation technologies from 20 km
platforms.
(g) Computer model at approximately 10 times the resolution of 2011
capability with approximately 7 km horizontal cell size.
(h) Reafﬁrm environmental buy-in with preliminary ﬁeld results. Further
dialogue with disparate communities.
4. Mini tests at 20 km. One per cent full scale (10 000 tonne p.a. or 200 tonne
per week): approximately 6 years £80 million p.a.
(a) With UN support, test effect of plume to 500–2000 miles lengths over
oceans: atmospheric chemistry, solar scattering, precipitation.
(b) Implement ﬁve 20 km altitude sampling points and three base stations.
(c) Monitor impact on a baseline series of ‘most sensitive areas’, e.g. South
Asian monsoon, Sahel and Amazonian precipitation.
(d) Examine precipitation and vegetative impacts as well as microbiology
under plume.
5. Five per cent scale at 20 km. 50 000 tonne p.a. £200 million p.a.
approximately 6 years.
(a) With UN support, increase to just-detectable effects over whole planet.
(b) Monitor ozone levels and regional precipitation.
(c) Modelling to 100 times the resolution of current capability (approx.
500 m resolution).
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6. Implementation only if mandated by UN Security Council. Allow four years
to build facilities and then ramp up full implementation in 10 years. Cost
approximately £500 million p.a. at the start, rising to £3 billion p.a. at
the end (2011 prices) for complete scaled-up system, including the cost
of a manufactured particle such as 1 Mt p.a. titanium dioxide (approx.
15% of current world production rates). It is assumed that costs of TiO2
are comparable to 2011 prices (around £2000 per tonne), but they could
be lower because of the scale of the operation or higher because of an
increased demand. At a rate of 2.5 Mt p.a., titanium dioxide costs would
increase to £6–7 billion p.a. and would account for around 25 per cent of
world titanium dioxide production but it is hoped that these rates would
not be needed.